In accordance with the recent miniaturization of design rules due to higher integration, a particle size to be assuredly measured by laser surface inspection devices is also miniaturized. During this measurement, if a haze level of an epitaxial wafer is high, the haze becomes a noise component for particle measurement and interferes with the measurement of minute-size particles. Thus, a silicon epitaxial wafer used as a substrate for a highly-integrated semiconductor device requires reduction of the haze level.
Further, in accordance with the miniaturization of semiconductor devices, high flatness becomes required for wafers. An area where flatness is to be assured on a wafer surface also tends to extend, and users become to often demand the assurance for the flatness substantially over the whole surface of a wafer, that is, over an area from the center of a wafer to 2 mm inwardly away from the outer periphery thereof.
With respect to the flatness of wafer, particularly the edge portion of wafer is apt to sag to be below the central portion, and thus decreases an available region for device fabrication, thereby leading up to deterioration of device fabrication yield. This sagging phenomenon is called “Edge Roll-off”.
Also for epitaxial wafers, the problem of this edge roll-off is important from the viewpoint of the device fabrication yield. To minimize the edge roll-off and to maintain satisfactory flatness is required for the epitaxial wafers. It should be noted that the edge roll-off includes not only a case where the edge portion of the wafer sags but also a case where the edge portion becomes higher than the center, depending on conditions.
Further, what to be noted regarding the flatness of epitaxial wafer is orientation dependence that the growth rate of epitaxial layer depends on crystal orientation of wafer. This is a phenomenon that, when <011> crystal orientation extending from the center of a wafer to the outer periphery thereof is taken as the reference as being 0°, the growth rate is high in orientations of 90°, 180°, 270° and 360° (namely, 0°), and the film thickness of epitaxial layer (hereinafter simply referred also to as “epitaxial film thickness”) is increased.
The epitaxial film thickness is increased in these four orientations (or axially symmetrically), and recessed portions (valleys) are formed between neighboring orientations among them. Accordingly, this phenomenon can be called also “4-Fold Symmetry”. Hereinafter, this orientation dependence of epitaxial growth rate is also simply called “orientation dependence of growth rate”, and the “4-Fold Symmetry” is called “4FS” for short.
For example, in an epitaxial wafer of 300 mm or more in diameter, this orientation dependence of growth rate (4FS) prominently emerges, particularly, from a 148-mm radial position, and is much more intensified nearing the outer peripheral zone (the chamfer vicinity).
In the epitaxial wafer production that epitaxially grows silicon on a silicon substrate, a chemical vapor deposition (CVD) method is mainly used from the viewpoint of the crystallinity of epitaxial growth layer, mass productivity, simplicity of the apparatus and the like. In the CVD method, a source gas containing silicon (Si) is introduced into a reactor together with a carrier gas (generally H2), and Si, which is generated by thermal decomposition or reduction of the source gas, is deposited as an epitaxial layer on the silicon substrate heated to a high temperature.
Examples of the Si-containing source gas (silicon source) include four gases of silicon tetrachloride (SiCl4), trichlorosilane (SiHCl3), dichlorosilane (SiH2Cl2), and monosilane (SiH4). As industrially-used source gases, silicon tetrachloride and trichlorosilane are mainly used, and dichlorosilane is also partially used, while monosilane, which allow low-temperature growth, is also partially used.
The epitaxial growth rate is dependent on kinds of source gases, temperature, pressure, and the like. A temperature region, in which the epitaxial growth can be performed, (hereinafter simply referred also to as “growth temperature region”) is qualitatively divided into two territories of reaction dominant one and feed (diffusion) dominant one. The reaction dominant territoy is located on a low-temperature side within the growth temperature region. In the reaction dominant territory, the growth rate becomes higher with temperature becoming higher.
On the other hand, the feed (diffusion) dominant territory (hereinafter referred to as “feed dominant territory”) is located on the high-temperature side within the same temperature region and is a region where temperature dependence is small, and the epitaxial growth is generally performed in this feed dominant territory. When the source gas is trichlorosilane, the epitaxial growth temperature is not lower than 1100° C. When the source gas is dichlorosilane, the epitaxial growth temperature is not lower than 1000° C., which is lower than in case of trichlorosilane.
In the current production of epitaxial wafer of 300 mm in diameter, trichlorosilane is used as source gas from the viewpoint of higher growth rate, and 1100 to 1130° C. within the feed dominant territory is used as the growth temperature region. The haze level in this case is about 0.12 to 0.18 ppm, which is measured in DWN mode using a particle counter SP-1 of KLA-Tencor (hereinafter simply referred to as “measured in DWN mode), and the haze is likely to become the noise component when the measurement is made for particles as being 35 nm or more in size that is a target in a next-generation assurance Standard.
Further, the orientation dependence of growth rate (4FS) is obtained as described below. A <011> orientation is taken as a reference, an epitaxial film thickness along this reference orientation is characterized and this determined thickness is taken as 1. On the other hand, an epitaxial film thickness along an orientation of 45° relative to the reference orientation is obtained and this obtained film thickness is converted into a relative film thickness with respect to the reference film thickness (along the reference orientation). Then, the orientation dependence of growth rate (4FS) is obtained and is about 0.980.
Namely, the difference of the thickness (this difference is herein referred to as “gap intensity”) is 0.020, or about 2% when expressed in percentage, and further reduction of the difference is desirable for maintaining satisfactory flatness of the epitaxial wafer. This gap intensity is not affected so much by the epitaxial growth rate and/or growth temperature.
To date, studies and developments on the production of epitaxial wafers with low haze level have been made. For example, Patent Literature 1 proposes a method, in which a growth temperature during epitaxial growth is set lower than an ordinary growth temperature by about 50 to 100° C.
Concretely, as shown in examples thereof, there is a description that, when trichlorosilane is used as source gas, a haze level shows a substantially minimum value under a growth temperature in the range of not lower than 950° C. to not higher than 1050° C. This allows the improvement in measurement accuracy with a particle counter and production of epitaxial wafers with minimized haze and good quality.
According to the method described in Patent Literature 1, the haze level can be substantially reduced, compared with a case in which general growth temperature (1100 to 1130° C.) is adopted. However, the growth temperature adopted therein is within the reaction dominant territory where temperature dependence of epitaxial growth rate is large, and it is considered that accurate control of the epitaxial film thickness is difficult. The minimum haze level described in the examples of the same literature is 0.5 ppm, which is a result of measurement in DWO mode by a KLA-Tencor particle counter (SP-2), and this value is not necessarily a low level.
With respect to the flatness of the epitaxial wafer, Patent Literature 2, for example, bases a notion such that uneven film thickness of epitaxial layer predominantly causes the deterioration of flatness of epitaxial wafer. Thus, Patent Literature 2 discloses a measurement method for epitaxial wafer. According to this method, the film thickness is measured by an FT-IR method (a method using a Fourier transform infrared spectrophotometer) at plural points for measuring film thickness, allocated on a wafer surface; the measured film thickness values are used to calculate a flatness for each site (a wafer surface is divided into unit areas each having a specific shape, the unit area being a site); each flatness is compared with reference flatness information; and thereby the quality of flatness is determined for each site of the epitaxial layer. In this case, SFQR (the difference between maximum height and minimum height as viewed from reference surface) can be adopted as an evaluation index of site flatness.
Although Patent Literature 2 does not refer to the above-mentioned orientation dependence of growth rate (4FS), it can be sufficiently predicted that the 4FS affects the flatness, expressed in SFQR, of outer peripheral sites (outer peripheral areas).